The expression “on-line generation” of data during a reaction is used herein to denote generation of the data sufficiently rapidly that the data is available essentially instantaneously for use during the reaction. The expression generation of data “in on-line fashion” during a reaction is used synonymously with the expression “on-line generation” of data during a reaction. Generation of data from at least one laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction if the laboratory test consumes so much time that parameters of the reaction may change significantly during the test. It is contemplated that on-line generation of data can include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
Advances in polymerization and catalysis have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization-type (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Regardless of these technological advances in the polyolefin industry, common problems, as well as new challenges still exist. For example, the tendency for a gas phase process to foul and/or sheet remains a challenge, which can particularly be dependent on the polymer being produced and the catalyst system employed.
Fouling, sheeting and/or static generation in a continuous gas phase process, in for example heat exchangers, distributor plates, and probes, can lead to the ineffective operation of various reactor systems. In a typical continuous gas phase process, a recycle system is employed for many reasons including the removal of heat generated in the process by the polymerization reaction, and recycle processes offer many opportunities for fouling.
Evidence of, and solutions to, various process operability problems, including fouling, sheeting, chunking, agglomerating and static build up, have been addressed by many in the art. For example, U.S. Pat. Nos. 4,792,592, 4,803,251, 4,855,370 and 5,391,657 all discuss techniques for reducing static generation in a polymerization process by introducing to the process for example, water, alcohols, ketones, and/or inorganic chemical additives; PCT publication WO 97/14721 published Apr. 24, 1997 discusses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor; U.S. Pat. No. 5,066,736 and EP-A1 0 549 252 discuss the introduction of an activity retarder to the reactor to reduce agglomerates; EP-A1 0 453 116 discusses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates; U.S. Pat. No. 4,012,574 discusses the addition of a surface-active compound, a perfluorocarbon group, to the reactor to reduce fouling; U.S. Pat. No. 5,026,795 discusses the addition of an antistatic agent with a liquid carrier to the polymerization zone in the reactor; U.S. Pat. No. 5,410,002 discusses using a conventional Ziegler-Natta titanium/magnesium supported catalyst system where a selection of antistatic agents are added directly to the reactor to reduce fouling; U.S. Pat. No. 3,470,143 describes a reduction in fouling in mostly slurry processes for producing primarily elastomers using a fluorinated organic carbon compound.
Likewise, further evidence of, and solutions to, various process operability problems have been addressed by many in the art. For example, U.S. Pat. No. 3,082,198 discusses introducing an amount of a carboxylic acid dependent on the quantity of water in a process for polymerizing ethylene using a titanium/aluminum organometallic catalysts in a hydrocarbon liquid medium; U.S. Pat. No. 3,919,185 describes a slurry process using a nonpolar hydrocarbon diluent with a conventional Ziegler-Natta-type or Phillips-type catalyst and a polyvalent metal salt of an organic acid having a molecular weight of at least 300; U.S. Pat. No. 5,990,251 relates to increasing catalyst activity of a Ziegler-Natta-type catalyst by using very small quantities of a halogenated hydrocarbon, specifically a molar ratio between 0.001 and 0.15 of the halogenated hydrocarbon, particularly chloroform, to the metal of the catalyst, specifically titanium; U.S. Pat. No. 6,455,638 is directed to a polymer blend having components with different ethylene content, and U.S. Pat. No. 5,624,878 relates primarily to the use in polymerization of catalytic derivatives of titanium (II) and zirconium (II) metallocene-type complexes; both U.S. Pat. Nos. 6,455,638 and 5,624,878 mention generally, in passing, using in polymerization various solvents such as straight-chain hydrocarbons, cyclic and alicyclic hydrocarbons, perfluorinated hydrocarbons, aromatic and alkyl-substituted aromatic compounds, and mixtures thereof. U.S. Pat. No. 6,534,613 describes using a Ziegler-Natta-type catalyst in combination with a halogenated hydrocarbon, particularly chloroform, and an electron donor to produce polymers useful for making better quality films. EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature. U.S. Pat. No. 3,056,771 discloses polymerization of ethylene using TiCl4/(Et)3Al in a mixture of heptane and perfluoromethylcyclohexane, presumably at room temperature.
U.S. Pat. No. 5,352,749, to DeChellis et al, teaches that there are limits to the concentrations of condensable gases, whether ICA materials, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. The above-cited U.S. Pat. No. 5,352,749, and U.S. Pat. Nos. 5,405,922 and 5,436,304, disclose upper limits of ICA in the reactor are discussed, depending on the type of polymer being produced. U.S. Pat. No. 5,352,749 discloses that a limiting concentration of ICA (isopentane) exists, beyond which the reactor contents suddenly loose fluidization. The authors characterized this limit by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane was increased, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They therefore determined that this ratio (0.59) was a point of no return, below which the reactor will cease functioning due to loss of fluidization.
As described herein, attempts to operate polymerization reactors with excessive ICA concentrations cause polymer particles suspended in the fluid bed to become cohesive or “sticky,” and in some cases cause the fluid bed to solidify in the form of a large chunk. This stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting in the straight sided reaction section, sheeting in the dome of such a reactor or chunking, any of which can lead to reactor shut-downs, which in large scale reactors are expensive. These solid masses (sheets or chunks) of polymer eventually become dislodged from the walls and fall into the reaction section and settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning. The term “discontinuity event” is used to describe a disruption in the continuous operation of a polymerization reactor caused by sheeting, chunking or distributor plate fouling. The terms “sheeting and/or chunking” while used synonymously herein, may describe different manifestations of problems caused by excessive polymer stickiness in the fluid bed. In either manifestation (sheeting or chucking) the excessive polymer stickiness can lead directly to a reactor discontinuity event with the associated loss production.
Others have addressed stickiness prevention in gas phase reactors including U.S. Pat. Nos. 5,510,433, 5,342,907, 5,194,526 and 5,037,905. These patents disclose that very low density, sticky materials can be produced in gas phase reactors by adding 10-20 wt % of inert, “refractory” material to the fluid bed. Suitable refractory materials are micro-fine silica and carbon black. However, application of the technology is expensive and requires substantial investment in powder handling equipment in the production plant.
Stable operation of fluidized bed reactors used in the production of polymers requires the avoidance of conditions that lead to sticky polymer. Sticky, or cohesive polymer causes a range of problems in the gas phase reactor systems. For example, sticky polymer can reduce the quality of fluidization that occurs within the reactor, and can reduce the degree of internal mixing below the minimum levels required to disperse the catalyst and maintain stable temperature control. In addition, stickiness of the polymer can lead to the deposition of polymer product on the walls of the reactor expanded section, which often leads to the formation of dome sheets (solid masses of polymer material deposited on the walls of the “dome”, or expanded section of the reactor) In many cases, these dome sheets are large and massive, containing as much as 100 kg of agglomerated polymer. These dome sheets eventually fall from the dome and become lodged on the distributor plate, where they interfere with fluidization. In some cases, the dome sheets block the product discharge port, and force a reactor shut-down for cleaning. For these reasons it is desirable to have means of preventing excessive stickiness of the polymer product.
Polymer stickiness is thought to be a function of several process and product variables within the reactor. The relevant process variables include the reaction temperature and the concentrations (or partial pressures) of condensable components such as 1-hexene and isopentane in the reactor gas phase. In general, stickiness of the polymer is promoted by higher reaction temperature and higher condensable concentrations. Important product properties include the resin density, molecular weight (or melt index), and the molecular weight distribution (MWD). In general, stickiness of the polymer is promoted by lower resin density, lower molecular weight (higher melt index), and broader molecular weight distribution (Mw/Mn=MWD).
Fluid bed reactors used to produce polyethylene resin are normally operated with a relatively high reaction temperature. For example, in the production of a typical low density film resin (0.917 g/cc density, 1 dg/min melt index) produced with metallocene or Ziegler-Natta catalyst, the reaction temperature is typically operated at 85° C. A relatively high reactor temperature provides for a relatively high temperature differential over the cooling water temperature (which typically operates at 30 to 35° C.). This, in conventional practice, is thought to provide for maximum heat removal capability for maximum production rates.
It would be desirable to have a polymer production process that is free of polymer agglomeration or stickiness. It would also be desirable to have a process that allows higher concentrations of condensables and/or higher dew point temperatures in the reactors for higher production rates.
Our findings indicate that, in many cases, the operating temperatures are too high relative to the polymer sticking temperature. Although it appears counterintuitive, we found that it is possible to reduce operating temperatures and actually increase maximum production rates, while avoiding problems of resin stickiness.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. The most recently produced quantity typically undergoes mixing with previously produced quantities of the product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the expression “diluent” (or “condensable diluent” or “condensable diluent gas”) denotes condensable gas (or a mixture of condensable gases) present in a polymerization reactor with polymer resin being produced. The diluent is condensable at the temperatures encountered in the process heat exchanger. Examples of diluents include induced condensing agents (ICAs), comonomers, isomers of comonomers, and combinations thereof.
The expression “dry polymer resin” (or “dry version” of polymer resin) is used herein to denote polymer resin that does not contain substantial amounts of dissolved gas. An example of dry polymer resin is polymer that had been previously produced in a polymerization reactor and then purged to eliminate all (or substantially all) unreacted comonomers and ICAs that had been dissolved in the polymer at the time of production. As will be discussed herein, a dry version of polymer resin has significantly different melting behavior than would the same polymer resin if it were in the presence of a significant amount of condensable diluent gas and comonomer.
The expression polyethylene denotes a polymer of ethylene and optionally one or more C3-C10 α-olefins while the expression polyolefin denotes a polymer of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index.
One commonly used method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, during operation to produce polyolefins by polymerization, contains a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, catalyst, and (optionally) catalyst modifiers. Typically, any of several process control variables can be controlled to cause the reaction product to have desired characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of a catalyst. This gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of polymer that cannot be removed as product. Further, such chunks or sheets may fall onto the distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown. Prevention of such stickiness has been accomplished by controlling the temperature of the fluid bed to a temperature below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness, which in turn can, if left unchecked, may lead to the above conditions.
It is understood that the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone. Since the exothermic heat generated by the reaction is directly proportional to the rate of polymer production. In steady state operation of the reaction process, the rate of heat removal from the fluidized bed must equal the rate of rate of heat generation, such that the bed temperature remains constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficient to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems of polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point can be calculated knowing the gas composition and is thermodynamically defined using an equation of state.
Contrary to this belief, as suggested by Jenkins, et al. in U.S. Pat. No. 4,543,399 and related U.S. Pat. No. 4,588,790, a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The resulting stream containing entrained liquid is then returned to the reactor without causing the aforementioned agglomeration and/or plugging phenomena (which had been expected prior to Jenkins). The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation in a gas phase polymerization process.
The above-cited U.S. patents to Jenkins et al. suggest that when a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Consequently, a substantial increase in space-time yield, the amount of polymer production in a given reactor volume, can be achieved by condensed mode operation with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in “condensed mode” operation remains entrained or suspended in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In the process described by Jenkins, et al., vaporization occurs when the two-phase mixture enters the fluidized bed, with the (warmer) resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed. The heat removal capacity is further enhanced in condensed mode operation by the lower gas temperatures of the gas stream entering the fluidized bed. Both of these factors increase the overall heat removal capability of the system and thereby enable higher space-time yields (higher reactor production rates per unit volume of the fluidized bed).
Jenkins, et al. illustrate the difficulty and complexity of such reactor control in general, and of trying to extend the stable operating zone to optimize the space time yield in a gas phase reactor, especially when operating in condensed mode.
The cooling capacity of recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option described is to add non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs). Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, an ICA compound is selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. ICA materials include hexane, isohexane, pentane, isopentane, butane, isobutane and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
Two articles by Process Analysis & Automation Limited (PAA), entitled “Agglomeration Detection by Acoustic Emission,” PAA Application note: 2002/111 (©2000) and “Acoustic Emission Technology—a New Sensing Technique for Optimising Polyolefin Production” (©2000), suggest process control in fluidized bed production of polyolefins utilizing acoustic emission sensors located at various positions on the reactor and recycle piping.
PCT Application Publication Number WO 03/051929 describes the use of mathematical chaos theory to detect the onset and presence of sheeting in a fluid bed reactor. Signals from a range of instruments, including acoustic emission sensors, differential pressure sensors, static sensors, and wall temperature sensors are filtered by certain specified methods to construct a “time-series” of data, which is then processed by methods of non-linear dynamics herein referred to as chaos theory and compared to data from a control reactor running without sheeting. The onset of sheeting is indicated by an increase in mean “cycle time” (relative to a baseline, control reactor), usually with a concurrent decrease in the “mean deviation” of the time-series. Alternatively, the onset of sheeting is indicated by a decrease in the mathematical “entropy” of the time-series data, as compared to a similar reactor running without sheeting. (The terms “time-series”, “cycle time”, “mean deviation”, and “entropy” here refer to calculated parameters defined by chaos theory.)
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor), e.g. 50 psia, while other polymers may tolerate as little as 5 psia. In these latter polymers, the heat transfer limited production rates under similar conditions are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Metallocene catalyst produced polymers are a good example of polymers with such a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type, including reactor temperature, comonomer type and concentration. Further, with the effect of temperature, ICA level and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has heretofore been difficult.
Even within the constraints of conventional, safe operation, control of such reactors is complex adding further to the difficulty and uncertainty of experimentation if one wishes to find new and improved operating conditions that might result in higher production rates. Large-scale gas phase plants are expensive and highly productive. Risks associated with experimentation in such plants are high because downtime is costly. Therefore it is difficult to explore design and operating boundaries experimentally in view of the costs and risks.
It would be desirable to provide a method of determining a stable operating condition for gas fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and the determination of desirable process conditions for optimum or maximum production rates in a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness that is a better or earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators to determine when conditions of limiting stickiness are being approached, and enable them to take corrective action before discontinuity events (such as sheeting and chunking) occur, while keeping the reactors at or near conditions of maximum ICA concentration, permitting higher production rates with substantially less risk.
U.S. patent application Ser. No. 11/227,710, entitled Method for Operating a Gas-Phase Reactor at or Near Maximum Production Rates While Controlling Polymer Stickiness, filed by Michael E. Muhle and Robert O. Hagerty on Sep. 14, 2005, discloses monitoring (during operation of a polymerization reactor) of resin stickiness by generating a time series of readings of acoustic emissions of the contents of the reactor using acoustic emission sensors. Acoustic emission measurements are generated during steady state operation of a reactor (producing the relevant polymer). Additional acoustic emission measurements (generated during operation of the reactor) are then processed to determine whether they deviate from acoustic emissions indicative of steady state reactor operation. Such deviation is treated as an indication of onset of excessive stickiness of polymer particles in the reactor. Corrective action can be taken (e.g., ICA and/or monomer levels and/or reactor temperature can be adjusted) when the acoustic emission measurements are determined to deviate from those of a steady state reactor.